An electric storage cell having a casing made of a thin-film material such as a laminate film has excellent space efficiency and heat radiation. Therefore, such an electric storage cell is used in a secondary battery such as a lithium ion battery as well as an electric double layer capacitor.
In such a kind of the electric storage cell, the casing is formed, typically, through a press work. Therefore, as seen in a focal manner, portions subjected to a strong tensile or bending strain are distributed in places. Such portions are susceptible to a wrinkle or a crack causing liquid leakage.
JP2004-55171A discloses a reinforcing member provided in a corner of an inner surface of a concave portion of the film exterior.